Treatment of fibrous materials

ABSTRACT

In order to improve the surface characteristics of carbon fibers, e.g. their adhesion to a resin matrix, they are treated by immersion in an aqueous oxidising agent, e.g. a hypochlorite.

United States Patent Wells etal.

[ 3,657,082 [451 Apr. 18, 1972 TREATMENT OF FIBROUS MATERIALS Henry Wells, Wantage; William James Colclough, Didcot, both of England United Kingdom Atomic Energy Authority, London, England Filed: Dec. 27, 1968 Appl. No.: 787,558

Inventors:

Assignee:

Foreign Application Priority Data Jan. 3, 1968 Great Britain ..525/68 U.S. Cl ..=204/130 Int. Cl. ..C0ld 7/34 Field ofSearch ..204/130, 95, 173, 132, 128,

Primary ExaminerJ0hn H. Mack Assistant ExaminerR. L. Andrews Attorney-Larson, Taylor and Hinds [5 7] ABSTRACT In order to improve the surface characteristics of carbon fibers, e.g. their adhesion to a resin matrix, they are treated by immersion in an aqueous oxidising agent, e.g. a hypochlorite.

2 Claims, No Drawings TREATMENT OF FIBROUS MATERIALS The present invention relates to the treatment of fibrous material and more particularly to the treatment of carbon or graphite fibers.

It will be known that carbon fibers may be made by taking fibers of an organic material, such as polyacrylonitrile, and subjecting them to high temperatures. Very often the heat treatment is effected in several steps in different atmospheres and the final temperature may be in the range l,500 to 2,600 C so that the fibers may be at least partially graphitized. However, for convenience, the term carbon fibers is used inclusively hereinafter.

These carbon fibers may have many uses, but one use is to reinforce a matrix, more particularly a matrix of plastics material, in order to produce a reinforced or laminated material in rather a similar way to that in which glass fiber is used to reinforce plastics material. One difficulty that has been found is that poor adhesion may occur between the carbon fibers and the matrix which leads to reduced mechanical strength of the composite.

In order to overcome the above difficulty, it has hitherto been proposed to apply a surface oxidation treatment to the carbon fibers after production, but we have found that if this treatment is carried out in air or oxygen it is extremely liable to cause runaway oxidation and/or pitting, either of which materially weaken the carbon fibers. It has therefore been proposed in our co-pending British application Ser. No. 524/68 of even date herewith to effect the oxidation in a gaseous medium containing an inhibitor.

it is an object of the present invention to provide a process for improving the surface characteristics of carbon fibers thereby to improve the adhesion between them and a matrix, particularly a matrix of plastics material.

According to the present invention carbon fibers are surface treated by immersion in an aqueous oxidizing agent.

Preferably the aqueous oxidizing agent is a hypochlorite solution containing available chlorine.

One very desirable way of obtaining the hypochlorite solution is to generate it by electrolysis of a chloride solution using the carbon fibers as one electrode.

It may also be convenient to pre-clean the carbon fibers by treatment with a conventional cleaning solvent in order to remove any gross surface contamination.

In both the pre-cleaning stage and the aqueous oxidizing stage it is very desirable to make use of ultrasonic agitation in order to ensure good contact between the relevant liquid and the fibers.

In order that the present invention may more readily be understood an embodiment of the same will now be described by way of example.

In this embodiment a 15 gm batch of carbon fibers, each approximately 7.5 microns in diameter and 33 cms. long was immersed in a bath of trichlorotrifluoroethane and ultrasonic agitation at 13 kHz was applied for 10 minutes. The fibers were then removed from the bath and dried in a cool oven to remove all traces of the solvent.

The fibers were then placed in a bath containing 5 percent by weight of sodium chloride and 0.25 percent by weight of sodium hydroxide in water. The fibers were connected to the anode and were at the same time subjected to ultrasonic agitation at 13 kHz. A copper cathode was also inserted in the bath and current was passed between the cathode and anode at a current density of 2 amps per square inch of carbon fiber surface and at a voltage of 12 volts. After treatments for 5 minutes in this bath (which generated free chlorine) the carbon fibers were removed, rinsed in water and dried.

The carbon fibers were then incorporated into an epoxide resin matrix in the proportion of 55 percent by volume and the resulting composite was tested in a 3 point bend test on a 1 inch beam. The interlaminar shear strength of the composite was 6,275 p.s.i. which compares with a strength of 2,000 to 3,000 p.s.i. for a similar laminate made from untreated fibers.

We claim: l. A method of surface treating carbon fibers comprising generating a hypochlorite solution containing available chlorine by electrolysis of an aqueous chloride solution using carbon fibers as the anode.

2. The method of claim 1 wherein prior to said electrolysis the carbon fibers used as the electrode are pre-cleaned by solvent cleaning. 

2. The method of claim 1 wherein prior to said electrolysis the carbon fibers used as the electrode are pre-cleaned by solvent cleaning. 